1. Field of the Invention
The present invention relates to an induction heating type fixing device of an image forming apparatus.
2. Description of the Related Art
An electrophotographic type image forming apparatus is generally provided with a fixing device for fixing a toner image transferred onto a recording material such as a paper sheet by applying heat and pressure. As a configuration of the fixing device, a heating method using a ceramic heater or a halogen heater has been conventionally used in many cases. In recent years, however, an electromagnetic induction heating method has been used from a viewpoint of advantages of capability of rapidly generating heat, and the like.
A control of the electromagnetic induction heating type fixing device is performed by driving a switching element for supplying a high-frequency electric current to an excitation coil provided arranged in the fixing device with a driving signal of a pulse-width modulation (PWM) signal. An electric power control is performed by changing a driving frequency of the PWM signal in a frequency range equal to or higher than a resonant frequency (resonance point) which is determined by capacitance of a resonant capacitor within an electric power source and inductance of the excitation coil of the fixing device. There is a technique available for performing electric power control by adjusting a PWM driving frequency so that electric power becomes a maximum value set by a central processing unit (CPU) at the time of warm-up (from when the power was turned on until when temperature reaches a set value of temperature control), and when a target temperature is reached, keeping the temperature constant by changing the PWM driving frequency (e.g., Japanese Patent Application Laid-Open No. 2000-223253).
In the control of the electromagnetic induction heating type device using the PWM control, a relationship of an input power PW of the power source varies according to a PWM driving frequency f as illustrated in FIG. 12. More specifically, it has a characteristic in which, a maximum electric power PWp is supplied when a driving frequency is at a resonant frequency fpy, and an electric power is reduced when the frequency changes to a high-frequency side or a low-frequency side centered on the resonant frequency fpy. The electric power control can be performed by controlling the driving frequency f of the PWM driving signal by utilizing this characteristic.
The input power takes a maximum value at the resonant frequency fpy. Constants of the resonant capacitor and the coil within the fixing device are determined so that the resonant frequency fpy becomes 15 to 20 KHz. If a load inductance value of the fixing device is L1 and a capacitance value of the resonant capacitor is C1, the resonant frequency fpy is expressed by the following equation.fpy=½π√{square root over (L1×C1)}  [Equation 1]
A range of the driving frequencies of the PWM driving signals is generally 20 to 100 KHz, and it is used at frequencies equal to or greater than the resonant frequency fpy. There is a problem that the driving frequency enters into an audible field at equal to or less than 20 KHz, and it is felt as noise. Accordingly, a minimum driving frequency is set to 20 KHz. On the other hand, the maximum driving frequency is set to 100 KHz from a relationship of Radio Act of Japan. At the time of electric power control, if an electric power to be supplied to the excitation coil does not reach a target power PWo, the PWM driving signal continues to be driven in a state where the driving frequency of the PWM driving signals is a minimum frequency.
When a fixing roller serving as an electrically conductive heating element is made of an alloy having characteristics in which magnetic permeability is large at a low temperature, and the magnetic permeability becomes small with increase in temperature, an inductor value of a load becomes small when the fixing roller is at a high temperature. Therefore, when a temperature of the fixing roller becomes high, the characteristic of the fixing roller is changed, and the resonant frequency fpy becomes high. At this time, if the driving frequency remains constant, the driving frequency will become lower than the resonant frequency fpy after fluctuation. As a result, as illustrated in FIG. 12, a problem arises that the input power decreases, and a time until the temperature of the fixing roller reaches a target temperature becomes longer.
On the other hand, if the driving frequency is set high from the a state that the temperature of the fixing roller is low, in anticipation of a change in the resonant frequency, there is a problem that the target power cannot be supplied to the excitation coil at a low temperature, and the time until the fixing roller reaches the target temperature becomes longer.